Corn supplies in the U.S., the world’s biggest exporter, are declining at the fastest pace since 1996 just as a Midwest heat wave damages the world’s largest harvest for a third consecutive year.

Stockpiles were probably 3.168 billion bushels (80.47 million metric tons) on June 1, 47 percent less than on March 1, the average of 22 analyst estimates compiled by Bloomberg shows. The worst Midwest drought in more than a decade is wilting a harvest that the U.S. Department of Agriculture says will be the biggest ever. The agency updates its inventory estimate June 29 and its production forecast two weeks later.

Futures surged 25 percent since reaching a 20-month low June 15, and Morgan Stanley expects prices to advance another 10 percent to $7 a bushel in two months if the drought persists. The rally is boosting global food costs that the United Nations estimates dropped 14 percent from a record in February 2011 and widening losses for ethanol producers including Decatur, Illinois-based Archer Daniels Midland Co.

“We have a potential disaster developing for the U.S. corn supply,” said Peter Meyer, the senior director for agricultural commodities at PIRA Energy Group in New York who cut his corn- crop forecast after surveying fields in Illinois, Indiana and Ohio last week. “This year may be the worst yet.”